1. Field of the Invention
The present invention relates to a lithography method for processing large surface areas of microelectric devices. More specifically, the present invention provides a means for quickly processing large surface areas while maintaining high resolution.
2. Description of the Related Art
Microelectronic components are frequently processed using photolithography or electron beam lithography to define the surface area for which a milling, such as ion milling, etching, or doping operation will be subsequently performed. Such processes typically involve placing a shield over a surface of the components, and then removing portions of that shield located over sections wherein the subsequent operation is to be performed.
Photolithography typically utilizes ultraviolet light to define those portions of the protective covering to be removed, and to remain in place. The shielding substance is known as a photoresist. Photoresists come in two types: negative and positive. A positive photoresist degrades upon exposure to ultraviolet light, and a negative photoresist will undergo crosslinking of the polymer molecules within the photoresist upon exposure to UV light, so that the unexposed portions are easier to remove. Positive photoresists are more commonly used because they have superior gap resolution. The photolithography process involves spinning the polymer photoresist across the entire surface of the component to be processed, placing a mask with transparent and opaque portions over the photoresist, so that the desired portions of a positive photoresist (or undesired portions of a negative photo resist) are covered by the opaque portions. The photoresist is exposed to ultraviolet light through the mask, thereby causing a positive photoresist under the transparent portions of the mask to deteriorate, facilitating removal (or causing the portions of a negative photoresist under the transparent portions of a mask to undergo crosslinking of polymer molecules, facilitating removal of the unexposed photoresist). The mask is then removed, and a solvent capable of removing only the weaker portions of the photoresist is applied, thereby leaving photoresist over only those surfaces which must remain covered. The entire surface may then be subjected to a milling, etching, or doping process, with only the uncovered portions of the substrate actually undergoing the process. Although photolithography is a rapid process, the gap resolution attainable is limited, and is generally a function of the wavelength of the light used to affect the photoresist. For purposes of this application, the term gap resolution is defined as the ability to precisely expose only desired portions of the substrate. As light is traveling through the transparent portions of the mask, those portions of the light with a smaller wavelength will undergo greater refraction.
For creating extremely fine patterns, electron beam lithography is presently used. As in photolithography, electron beam lithography utilizes a substance known as a resist to cover a substrate, and then uses an electron beam to define those portions of the resist to be removed. A positive resist deteriorates upon exposure to the electron beam, and a negative resist hardens upon exposure to the electron beam, thereby facilitating removal of the unexposed portions. Electron beam lithography is frequently performed by directing the electron beam sequentially, on a pixel by pixel basis, across those portions of the resist for which exposure is desired. This is an extremely slow process.
Hybrid lithography is another presently available process, wherein both electron beam and optical lithography are performed using a single layer of resist. This process requires selection of a resist that is sensitive to both optical exposure and electron beam exposure, substantially limiting the available resist materials. Additionally, this process requires the use of equipment configured to expose the resist to appropriate, compatible wavelengths, greatly increasing equipment expenses.
Accordingly, there is a need for a lithography process having high resolution, but capable of being performed at a significantly faster rate.
The present invention is a lithography process, having the high resolution of electron beam lithography, which may be used to define narrow gaps, while providing the speed of photolithography for processing large surface areas.
The surface of a substrate for which lithography is desired is covered with an electron beam sensitive resist. The electron beam sensitive resist is covered with a photoresist. Both resists will typically be of the positive type, although negative resists may be used. A mask is placed over the photoresist, and the photoresist is exposed to radiation (most commonly ultraviolet light) through the transparent portions of the mask. The undesired portions of the photoresist are then removed by the appropriate solvent. At this point, a portion of the electron beam sensitive resist is shielded by the remaining photoresist.
An electron beam flood is applied to entire surface being processed. The undesired electron beam resist may then be removed by the appropriate solvent. Applying an optical flood to the entire surface removes the remaining photoresist, thereby exposing the entire surface of the remaining electron beam resist. At this point in the process, large areas of the substrate have been exposed with little processing time. Electron beam lithography may then be applied sequentially across the remaining electron beam resist, thereby permitting the creation of narrow gaps and precisely defined edges within the remaining resist. Because only a small portion of resist is removed through sequentially applied electron beams, the affect of this step on the overall processing time is minimized. At this point, the substrate may be subjected to a further processing, such as ion milling, etching, or doping process, with the electron beam sensitive resist protecting those portions of the substrate for which such processing is not desired.
This lithography process combines the advantages of other types of lithography while avoiding the disadvantages of the prior art. The lithography process of the present invention specifically utilizes process steps capable of being performed rapidly to expose large areas of the substrate, thereby reducing the time required for the overall process. Additionally, process steps capable of defining gaps and edges with high resolution are used only to expose smaller areas of the substrate, thereby increasing the precision of the overall process without substantially impacting the overall time necessary to complete the improved lithography process. Therefore, unlike prior lithography processes, the lithography process of the present invention does not require choosing between high resolution and rapid processing.
It is therefore an aspect of the present invention to provide a lithography process having high gap resolution.
It is another aspect of the present invention to provide a lithography process capable of being performed quickly.
It is a further aspect of the present invention to utilize electron beam floods, optical floods, and/or oxygen ashing to remove resists from larger surface areas, and electron beam lithography to remove resists from small surface areas.